This invention relates to a so-called extrusion coating device. More particularly, it relates to a coating device conveniently employed for coating magnetic paint for fabrication of a magnetic recording medium.
For an audio tape or a video tape, a so-called coated magnetic recording medium is employed, in which a magnetic layer is formed by coating a magnetic paint, produced on dispersing and kneading magnetic powders, binder, dispersant or lubricant in an organic solvent, on a non-magnetic support, such as a polyester film.
For producing the coated type magnetic recording medium, an extrusion system employing an extrusion die coating device for applying a magnetic paint on a non-magnetic support is attracting attention and put to practical use, although on a limited scale.
The extrusion coating device has a die having a slit of a larger width facing its forward end face and having its forward end portion formed as a doctor edge. The coating device is so designed that the magnetic paint continuously extruded onto the surface of a running non-magnetic support is applied by the doctor edge on the non-magnetic support to a uniform thickness. With the extrusion coating device, the paint is supplied from both sides of the die into a sink of the coating liquid provided on the back surface of the slit. The paint is supplied via the sink of the coating liquid into the slit and extruded via the forward end of the slit so as to be coated on the non-magnetic support.
Such extrusion coating device lends itself to reduction of size and production cost since it is readily capable of forming a coating film of a uniform thickness, and is not fitted with a roll of a larger diameter. In addition, the paint supplied into the die is applied in its entirety and need not be circulated about the die, so that it is employed in the field of photographic films or photographic papers.
Recently, in the coating type magnetic recording medium, a coating technique capable of sufficiently coping with the high-speed coating and reduction in thickness of the coating film for improving productivity has been desired.
To this end, the coating methods such as those shown in U.S. Pat. Nos. 4,424,762 or 4,480,583 or JP Patent Kokai Publication 60-238179, have been proposed in connection with the above-described extrusion coating devices.
With the coating devices, it is difficult to cope with the demand for high-speed coating or reduction in thickness of the coating film, such that, if the film thickness of the coating film is not more than 30 .mu.m in wet thickness, the produced coating film tends to have a fluctuating film thickness.
As a technique for overcoming such inconvenience, there has been proposed in, for example, JP Patent Kokai Publication 62-95170, a method in which a slit formed at the forward end of a die is thrust against the non-magnetic support, without supporting the back surface of the running non-magnetic support, and in which the paint is extruded via the slit by a permanent magnet of 50 to 5000 gauss on the opposite side of the slit in order to effect the coating.
However, the above method is not sufficient to respond to the demand for high-speed coating or reduction in thickness of the coating film. On the other hand, with the blade of a magnetic material, provided at the distal end of the die, the coating thickness along the width is marred in the produced coating film, such that an optimum coating film cannot be produced. In addition, a magnetic circuit is formed in which the magnetic flux of a permanent magnet is passed through the magnetic paint and returned back to the opposite pole of the magnet. Consequently, should the non-magnetic material be used as the blade material, the magnetic flux is scattered, although the effect proper to the magnet is derived. Thus the motion of the magnetic paint cannot be maintained during high-speed coating, such that coating cannot be performed with sufficient stability.
There is also proposed in, for example, the JP Patent Kokai Publication No. 4-214229 a method of employing an electro-magnet in place of a permanent magnet as blade magnetizing means.
However, since only the position of mounting the electro-magnet is defined with the present method by an angle the electro-magnet makes with the surface of a non-magnetic support, the above-mentioned inconvenience cannot be eliminated.
Thus it is not possible with the conventional coating device to achieve a sufficient effect in raising the coating speed and reducing the thickness of the coating film.